Subsurface Evolution and Persistence of Marine Heatwaves in the Northeast Pacific

Abstract

The reappearance of a northeast Pacific marine heatwave (MHW) sounded alarms in late summer 2019 for a warming event on par with the 2013-2016 MHW known as The Blob. Despite these two events having similar magnitudes in surface warming, differences in seasonality and salinity distinguish their evolutions. We compare and contrast the ocean's role in the evolution and persistence of the 2013-2016 and 2019-2020 MHWs using mapped temperature and salinity data from Argo floats. An unusual near-surface freshwater anomaly in the Gulf of Alaska during 2019 increased the stability of the water column, preventing the MHW from penetrating deep as strongly as the 2013-2016 event. This freshwater anomaly likely contributed to the intensification of the MHW by increasing the near-surface buoyancy. The gradual buildup of subsurface heat content throughout 2020 in the region suggests the potential for persistent ecological impacts.Plain Language Summary Surface marine heatwaves (MHWs) are periods of prolonged and extremely warm regional sea surface temperature that can negatively impact the health and productivity of marine ecosystems. Using surface and subsurface ocean observations, we compare and contrast two recent MHWs to show that salinity variations play an important role in the vertical distribution of temperature anomalies by changing the overall stability of the water column. During the 2019-2020 MHW, the near-surface waters in the Gulf of Alaska were fresher than normal, preventing warm sea surface temperatures from mixing as deeply into the subsurface as in the 2013-2016 MHW. The freshening in 2019 likely enhanced warming in the buoyant surface layer. As warmer temperatures gradually mix downward they can persist long after the surface MHW disappears, suggesting that the ocean can provide memory for long-lived MHWs. The subsurface persistence of MHWs has potential ramifications for long-lasting ecological impacts.

A Persistent and Intense Marine Heatwave in the Northeast Pacific During 2019–2020

“…Scannell et al. (2020) reported that the near‐surface freshwater anomaly and increased stability of the water column over the study area prevented the warm anomalies from penetrating as deep as the cases in 2013–2014 and 2015. In view of the intensity of temperature anomalies, the first peak in November 2019 was the strongest compared to the subsequent three peaks.…”

“…However, the characteristics of these MHWs in the boreal spring are not clearly demonstrated. Recently, significant warmer‐than‐normal SSTs have been observed over the NE Pacific from the spring of 2019 through the end of 2020 (Amaya et al., 2020; Scannell et al., 2020). The seasonal evolution of this persistent event and its differences from earlier NE Pacific MHWs should be investigated.…”

On the Vertical Structure and Propagation of Marine Heatwaves in the Eastern Pacific

“…To discard short extreme “heat spikes” from our further analyses, we applied morphological operations to the Boolean array B (Gonzalez & Woods, 2017), consisting of a binary closing followed by a binary opening in the temporal dimension (see Scannell, 2020 and Scannell et al., 2021, 2023). These morphological operations act to smooth the Boolean array by merging extremes that are briefly interrupted (turn intermediate “Falses” to “Trues”) and by removing temporally isolated, short extremes (turn “Trues” to “Falses”).…”

Column-Compound Extremes in the Global Ocean

“…The warm water mass advected into the Bering Sea in 2017-2018 (Basyuk & Zuenko, 2020;Stabeno & Bell, 2019), and may be represented by a peak in MHW-OAX-LOX in 2018 (Supporting Information Figure S11). Subsequently, lower sea-ice cover in the winter led to anomalous stratification and reduced vertical mixing from melting ice (Stabeno & Bell, 2019;Scannell et al, 2020), prolonging the lifetime of the CCX in the subsurface. In cluster 3, the largest event occurred in 1997, corresponding with the largest event of cluster 5, and a strong El Niño event (Supporting Information S11).…”

“…Detecting extremes across the vertical dimension is thus an important step towards understanding the compression of habitable space during such extremes. Some MHW studies have looked into the subsurface, (Schaeffer & Roughan, 2017;Elzahaby et al, 2021;Scannell et al, 2020;McAdam et al, 2022;Fragkopoulou et al, 2023), while the concept of habitat compression has been considered with respect to temperature and oxygen changes (Jorda et al, 2020;Köhn et al, 2022). However, a consistent definition of compound extremes in the column has yet to be defined.…”